The present invention relates to a matrix type liquid crystal display device, and particularly to the structure of a matrix type liquid crystal display device which permits an image to appear at an improved contrast.
In a conventional matrix type liquid crystal display device a minimum voltage is applied to electrode elements of each picture element (pixel) in the matrix so as to avoid cross-talk between adjacent pixels, thereby assuring independency in driving selected pixels. An increase of the minimum voltage however causes the reduction of contrast. It is well known that the most appropriate driving condition, namely the maximum ratio ".alpha." of the maximum value of rms voltage "Von" to the minimum value of rms voltage "Voff", to be applied to a given pixel in an "n"-ratio matrix is given by: EQU .alpha.=[(n.sup.1/2 +1)/(n.sup.1/2 -1)].sup.1/2.
If the ratio ".gamma." of the liquid crystal device is equal to or smaller than .alpha.[(.gamma./.alpha.).ltoreq.1], where .gamma.=V.sub.SATLC /V.sub.TLC V.sub.SATLC represents the optical saturation voltage of the liquid crystal device, and V.sub.TLC represents the threshold voltage of the liquid crystal device, a voltage to be applied to a given pixel can be varied in the range from the threshold voltage V.sub.TLC to the saturation voltage V.sub.SATLC, thus enabling the selection of applied voltage to a pixel most appropriately for increasing its contrast to a possible maximum.
If "n" increases, however, ".alpha." decreases to 1 accordingly, and as a result (.gamma./.alpha.)&lt;1. Then, the contrast cannot be improved with recourse to the appropriate setting the applied voltage. Thus, the maximum improvement of contrast ratio is decided by the value of (.gamma./.alpha.) only. The description so far relates to the contrast of a pixel in the matrix.
As is well known, there are some different types of liquid crystal display device such as twisted nematic type liquid crystal and guest-host type liquid crystal. These liquid crystal display devices may be again classified in transparent and non-transparent types. The former liquid crystal display allows light to pass therethrough when not subjected to an electric field (commonly called "normally white"), whereas the latter liquid crystal prevents light from passing therethrough when not subjected to an electric field (commonly called "normally black"). No matter which type liquid crystal is used, production of liquid crystal display matrixes of the same contrast cannot be assured. This is attributable to the interspace between adjacent electrode elements of picture elements.
The contrast of the "normally white" liquid crystal matrix is described with reference to that of the "normally black" liquid crystal matrix.
FIGS. 1A, 1B and 1C show the electro-optical characteristics of a twisted nematic liquid crystal display matrix. Ordinates represent the average spatial intensity of light passing through the liquid crystal display matrix whereas abscissas represent the rms values of voltage applied to the liquid crystal display matrix. Specifically, FIG. 1A shows the electro-optical characteristics of a "normally white" liquid crystal display device. As shown, the light transmittance "Ls" starts decreasing when the voltage increases beyond the threshold voltage "V.sub.TLC ", and the light transmittance "Ls" decreases to the minimum "Lo" when the voltage increases beyond the saturation voltage "V.sub.SATLC ". This minimum represents the amount of light passing through the spaces between adjacent pixels in the matrix, and the minimum light transmittance Lo depends on the design and arrangement of transparent pixels. As a matter of course the value of the minimum light transmittance increases with an increase in the space between adjacent pixels in the matrx. The contrast of the liquid crystal display matrix cannot be raised above (Ls/Lo). Therefore, no matter how large a value of contrast, individual pixels may have, the contrast of the whole liquid crystal display matrix remains at a relatively low value.
FIG. 1B show an electro-optical characteristics of a "normally black" liquid crystal display device, which prevents light from passing therethrough when not subjected to an electric field. As shown, almost no light can pass through the liquid crystal display device when the applied voltage remains at a relatively low value. The light transmittance rises sharply when the applied voltage increases beyond "V.sub.TLC ", and a maximum amount of light (Ls-Lo) is allowed to pass through the liquid crystal display matrix when the applied voltage increases beyond "V.sub.SATLC ". The maximum light transmittance is equal to (Ls-Lo) rather than Ls. This is attributable to the fact that even if the pixels turn transparent, the spaces between adjacent pixels prevent light from passing therethrough, thus making the total amount of light passing through the whole area of the liquid crystal display matrix smaller than that of the "normally white" liquid crystal display matrix by the ratio of (Ls-Lo)/Ls. From the point of contrast, however, the amount of light at "black" level is so small that the contrast of the whole liquid crystal matrix is raised to a relatively high value. As is apparent from the above, the contrast of a liquid crystal display matrix is of as much concern as the contrast of individual electrode elements. The present invention aims at the reduction of adverse effect caused by the bias light passing through the spaces between adjacent pixels on the contrast of the whole liquid crystal display matrix.
The "normally black" twisted nematic type liquid crystal display matrix has a defect of browning tendency. If the matrix digit "n" is large enough to cause the ratio (.alpha./.gamma.) to be smaller than one, the driving voltage to be applied to the liquid crystal matrix is selected around "V.sub.TLC " with a view to obtaining a best possible contrast. As a consequence the drive voltage remains at a relatively low voltage, and accordingly the responsiveness is lowered. In contrast to this, the "normally white" liquid crystal display matrix of FIG. 1A require the drive voltage to be around V.sub.SATLC for its best contrast. V.sub.SATLC is higher in value than V.sub.TLC and accordingly the responsiveness is fairly good.
FIG. 1C shows electro-optical characteristics of a "normally white" liquid crystal matrix display whose pixel to pixel spaces are covered with a non-transparent material according to this invention. As a result the amount of light passing through the spaces between adjacent pixels reduces from Lo to Loo, and therefore the total light transparence of the liquid crystal matrix is reduced from Ls to (Ls-Lo+Loo). The amount of bias light at "black" level, however, reduces from Lo to Loo. Thus, the ratio of contrast (Ls-Lo+Loo)/Loo increases to the extent that it is fairly close to the contrast of a "normally black" liquid crystal matrix. Still advantageously for a relatively large value of "n", and hence for .gamma./.alpha.&lt;1 the drive voltage to be applied to the liquid crystal matrix is around V.sub.SATLC, and accordingly the responsiveness is much better than that in a "normally black" liquid crystal matrix.